I think the simplest design is with a plastic milk jug.
Fill the jug about 1/3 or so with sand. Remember, you have to carry it. When you get to your spot, fill the rest of the jug with water. Take the decoy, with string attached, and run the string through the handle and sink the jug. Walk the string back to shore and jerk away.
The reasons I prefer the milk jug are: (1) it is free, (2) it has a convenient carry handle, (3) the handle is smooth and won’t shred the cord, and (4) at the end of the hunt, empty the sand and water out of the jug, so it weighs almost nothing to carry out.